Respiratory Fluid Mechanicsand Medical Ventilators

Thu Zar KyawLing Tsou

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Introduction• Anatomy and Physiology of Respiratory

System• Spirometer• The need for artificial respiration• Polio and Iron lung• Positive pressure ventilators• CPAP and PEEP• Classification of medical ventilators• The future of ventilators

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Respiratory System

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Upper RespiratoryTract

Nasal Cavity

Nasopharynx

Oropharynx

Laryngopharynx

Oral Cavity

Epiglottis

Vocal Cords (folds)

Trachea

Esophagus

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Lower Respiratory Tract

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Pressure Changes in Pulmonary Ventilation

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Lung Volume and CapacitiesAt rest: a healthy adult

averages 12 breaths per minute

Moving about 500mL of air into and out of the lungs.

The rate of the air moving in and out:

12breaths/min x 500mL/breaths = 6 L/min

Maximum exercise: a healthy adult averages 60 breaths per minute

Moving about 3000mL of air into and out of the lungs

The rate of the air moving in and out:

60 breaths/min x 3000mL/breaths = 180 L/min

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Spirometer

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One of the Applications of Spirometer

• The spirometer is widely used in measuring lung function

• Eg. Asthma and COPD: Asthma/COPD is a disease of the respiratory system in which the air ways constrict and increases airway resistance.

• From spirometer, the stage of patients can be known.

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The Need for Artificial Respiration

COPDAsthmaPneumoniaLung CancerSevere Acute Respiratory Syndrome (SARS)PoliomyelitisEtc.

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Medical Ventilator• Definition:

– A flow generator. A machine that generates a controlled flow of gas into a patient’s airways.

• Types:– Negative Pressure– Positive Pressure

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The use of ventilators

• Intensive care• During surgery• Home care• Life support

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Poliomyelitis

• Respiratory failure is a consequence of the destruction of motor cells in the spinal cord innervating respiratory musclesneuronal destruction of the respiratory center in the medullaboth.

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Iron Lung

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Iron Lung

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Iron Lung

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History of Iron Lung• Iron Lung was invented by Dr. Philip Drinker and

Dr.Louis Agassiz Shaw of Harvard Medical School.

• They injected the cat with the South American arrow-poison curare to produce an effect of extremely relaxed muscles, to the point of respiratory arrest.

• They then placed the cat into the sealed box and used a hand-operated piston to manually control the pressure.

• This experiment was successfully ventilated the cat for a few hours.

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From negative pressure to positive pressure

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Positive Pressure Ventilator

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CPAP & PEEP• CPAP – Continuous Positive Airway Pressure

• PEEP – Positive End Expiratory Pressure

Elevated baseline airway pressure

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Purpose of CPAP:To avoid low lung volume

• Functional Residual Capacity (FRC)– The lung volume after a normal expiration. – Equals to the residual volume + expiratory reserve volume.

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Classification of machine ventilations

1. Type of Control:- Volume Controlled- Pressure Controlled- Dual Controlled

2. Type of Cycling:- Time Cycled- Flow Cycled- Volume Cycled

3. Triggering:- Time, Pressure, or Flow.

4. Breathing:- Mandatory, Assisted, or Spontaneous.

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Phase 1 – Inhalation. The pressure increases until a peak pressure is reached.

Phase 2 – Pressure drops to a stable level, known as the plateau phase.

Phase 3 – Exhalation. The pressure falls to a residual value.

Three Phases of RespiratoryAirway Pressure

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Three Main Problems with Mechanical Ventilation

• Peak pressure may increase mechanical stress on the lung tissue.

• The residual pressure drops if breathing gas is able to escape through a leak. (constant residual pressure has great clinical significance - PEEP)

• Patient is unable to breathe out during the plateau stage. Natural breathing is considerably impaired during this phase.

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Pressure-Controlled Ventilationpressure limited

• Uses machine ventilation bellows.

• Enables ventilation to be performed with the airway pressure limited to the level of the “working pressure”.

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Pressure-Controlled Ventilationwith valves and microprocessor

• Substitutes the bellows with modern valves.

• Dynamic gas flow and expiratory valve control.

• Electromagnetic drive mechanism.

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Pressure-Controlled Ventilationwith open system

• The expiratory valve is controlled with great sensitivity.

• Allows spontaneous breathing.

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Volume-Controlled Ventilationwith intermittent mandatory ventilation

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Volume Assist Controlled

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Synchronized Intermittent Mandatory Ventilation

• Another method for spontaneous breathing.• Allow the patient to breathe spontaneously

while also receiving mandatory breaths.

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Pressure-Controlled

• The inspiratory flow and flow waveform are determined by the ventilator.

• Requires the clinician to titrate the inspiratory pressure to the measured tidal volume.

• Limits the maximum airway pressure delivered to the lung, but may result in variable tidal and minute volume.

Volume-Controlled

• Requires the clinician to appropriately set the inspiratory flow, flow waveform, and inspiratory time.

• Offers the safety of a pre-set tidal volume and minute ventilation.

• Airway pressure increases in response to reduced compliance, increased resistance, or active exhalation and may increase the risk of ventilator-induced lung injury.

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Pressure-Controlled

Volume-Controlled

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Volume Controlled+

Pressure Controlled

Synchronized Intermittent MandatoryVentilation with Pressure Support:

Pressure controlled SIMV mode

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ICU Ventilators

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The Future of Ventilators

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Reference• http://peer.tamu.edu/curriculum_modules/OrganSystems/module_4/howweknow.htm• http://historical.hsl.virginia.edu/ironlung/pg3.cfm• http://en.wikipedia.org/wiki/User:Richard_Hill• Principles of Anatomy & Physiology

11 Edition. Pages 846-890 Tortora & Derrickson• 2005-2006 respiratory Lecture, McMaster University.• Maire P Shelly, Peter Nightingale, “ABC of intensive care: Respiratory support”. October 2006. http://bmj.com• Chien Sheng Wang, Dein Shaw, Kuen Shan Jih, “An intelligent control system for ventilators”. September 1997.• “The Evolutionary Conceptual History of Oscillatory Demand Continuous Positive Airway Pressure (OD-CPAP)”• Ernst Bahns, “The Evolution of Ventilation”. Dragerwerk AG.• http://www.eml-eis.com/Medical/Ventilation/Home/Ventilation.htm

Electrical Instrumentation Services Ltd.• http://www.ccmtutorials.com/rs/mv/index.htm

Mechanical Ventilation Tutorials• Robert S Campbell RRT, Bradley R Davis MD Capt MC USAF, “Pressure-Controlled Versus Volume-Controlled Ventilation: Does It Matter?”.

http://www.rcjournal.com/contents/04.02/04.02.0416.asp• Tufts-New England Medical Center

http://www.nemc.org/RespCare/Policies.htm• Coast to Coast Medical, Inc

http://www.coast2coastmedical.com/ventilators.htm• Managing RSV “SPAG-2 User’s Guide”

http://www.rsvinfo.com/managing/spag2/spag2.html• Papapostolou Medical Equipment Center “Galileo & Galileo Gold”

http://www.papapostolou.gr/• University of Utah, University Hospitals & Clinics, Newborn Intensive Care Unit

http://uuhsc.utah.edu/wcservices/nbicu/• Dumont Masonic Home, “Ventilator and Respiratory Care Program”• http://www.trimsdumont.org/ventRespCare.cfm• University of California San Diego, School of Medicine

http://meded.ucsd.edu/• London Health Sciences Centre, Critical Care Trauma Centre, “Mechanical Ventilator”

http://www.lhsc.on.ca/critcare/icu/focis/words/ventil.html• Anne Thomson, “The role of negative pressure ventilation”. Department of Paediatrics, Oxford Radcliffe Hospital

http://adc.bmjjournals.com/cgi/content/full/77/5/454• Focus, News from Harvard Medical, Dental, and Public Health Schools, “Milestone Reached, But Campaign Against Polio Continues”

http://focus.hms.harvard.edu/2005/Mar25_2005/infectious_disease.shtml• Dacotah Prairie Museum

http://www.dacotahprairiemuseum.com/collection/i.html• Children’s Hospital Boston

http://www.childrenshospital.org/research/polio_gallery/photo9.html